The present invention relates to titanium-based carbonitride alloys particularly useful in tools for chipforming machining such as turning and milling.
It is generally known in the cemented carbide industry that surface defects such as cracks, pores, etc., near to a working cutting edge can have a negative influence on the efficiency of cutting tools. Cemented carbide usually means sintered hard alloys based upon tungsten carbide (WC), cobalt (Co) and cubic carbides such as TiC, TaC and NbC.
A group of materials being used today mainly for inserts for finishing operations are titanium-based carbonitride alloys colloquially often named cermets. The hard phase of these alloys consists essentially of cubic phases of so-called Bl-type of TiC-TiN being wholly or partly alloyed with other carbide- or nitride-forming elements such as W, Mo, Ta, Nb, V, Hf, Cr and Zr. The hard constituents are usually present as more or less rounded particles with core-rim structure but can also be present as needle- or disk-shaped monocrystals. By suitable choice of raw materials and/or manufacturing method the core-rim structure can be modified so that desired properties are obtained. The binder phase consists of one or more of the metals Fe, Co and Ni of the iron group, usually Ni or Ni+Co. Often the binder phase is alloyed with one or more of the carbide- or nitride-forming elements. Other hard phases other than the cubic nitrides and carbides can occur, e.g., WC. The mentioned alloys are in their nature considerably more brittle than the classic cemented carbide essentially due to the fact that the wetting between titanium hard constituents and the binder phase--consisting of either merely nickel or of nickel and cobalt--is not as good as that between tungsten carbide and cobalt.
Furthermore, it can be observed that these carbonitride alloys are seen relatively as more fine-grained than "normal" cemented carbide. This essentially depends upon the fact that the actual powder is more difficultly ground than that of cemented carbide, is coming from more fine-grained raw materials and/or has less disposition towards grain growth. Fine-grained powder is more difficult to press to pressbodies than less difficult ground powder because of, i.e., spring-back.
Titanium-based carbonitride alloys also often contain, intentionally or unintentionally, a surface zone with another composition than the rest of the material being up to some 100 .mu.m in width.
Hard constituents such as carbides, nitrides ad/or carbonitrides of titanium have a much greater thermal expansion coefficient than tungsten carbide. As the amounts of the hard constituents as well as those of the binder phase are about the same, the titanium carbonitride alloys have a considerably greater thermal expansion coefficient than ordinary cemented carbide. This causes a titanium carbide pressed body to expand more relatively than a cemented carbide pressed body during heating to sintering temperature.
From the above, and from other additional factors, it is realized that it is considerably more difficult to make dense bodies of carbonitride alloys being free of defects than of cemented carbide, i.e., because cracks or other weaknesses from the pressing have much greater tendencies to open during the run up to the sintering temperature. This is particularly applicable to complicated geometrical forms having sudden "steps" with relative thickness differences. As carbonitride alloys furthermore are brittle by their nature, a disastrous influence on the toughness behavior can be expected for sintered cutting inserts of carbonitride alloys if they have defects of the above mentioned kind near to a working edge.